1. Field of the Invention
The present invention relates to a golf ball having a cover formed from a thermoplastic polyurethane material; and more particularly to a golf ball having a cover formed through molding from a thermoplastic polyurethane material exhibiting excellent mold releasability.
2. Description of the Related Art
In recent years, polyurethane materials have become of interest as materials for forming a golf ball cover. Polyurethane materials are classified broadly into thermosetting polyurethane materials and thermoplastic polyurethane materials, and a process for forming a thermosetting polyurethane material into a product differs from a process for forming a thermoplastic polyurethane material into a product. A thermosetting polyurethane material can be formed into a product through the following procedure: a urethane prepolymer having an isocyanate end group and a curing agent such as polyol or polyamine, which serve as liquid raw materials, are mixed under heating; and the resultant mixture is fed directly to a mold and then heated, to thereby allow urethane curing reaction to proceed.
Many studies have heretofore focused on golf balls formed from thermosetting polyurethane materials. For example, U.S. Pat. Nos. 5,334,673, 6,117,024, and 6,190,268 disclose such golf balls. Meanwhile, U.S. Pat. Nos. 5,006,297, 5,733,428, 5,888,437, 5,897,884, and 5,947,843, among others, disclose forming methods of thermosetting polyurethane materials.
Since a thermosetting polyurethane material exhibits no thermoplasticity, the material and a product formed from the material cannot be recycled. In addition, when a thermosetting polyurethane material is employed for forming a specific product such as a golf ball cover (i.e., a product which covers a core), efficient production of the product is not attained, since the heating-curing step and the cooling step of the material requires long time, and high reactivity under heat and instability of the material make control of the molding time very difficult.
In the case where a thermoplastic polyurethane material is formed into a molded product, the product is not directly obtained through reaction of raw materials, but is formed from a linear polyurethane materialxe2x80x94an intermediatexe2x80x94which has been synthesized by employment of raw materials and a synthesis method, the raw materials and the method differing from those employed in the case of the aforementioned thermosetting polyurethane material. Such a linear polyurethane material exhibits thermoplasticity, and is cured through cooling. Therefore, such a polyurethane material can be molded by use of an injection molding machine. Injection molding of a thermoplastic polyurethane material is a technique best suited for forming a golf ball cover, since the molding time of a thermoplastic polyurethane material is much shorter than that of a thermosetting polyurethane material, and a thermoplastic polyurethane material is suitable for precise molding. Meanwhile, a thermoplastic polyurethane material can be recycled, and is thus environmentally friendly. U.S. Pat. Nos. 3,395,109, 4,248,432, and 4,442,282, among others, disclose golf balls formed from thermoplastic polyurethane materials.
However, golf ball covers formed from conventional thermoplastic polyurethane materials exhibit poor mold releasability during molding. In order to cope with such a problem, Japanese Patent Application Laid-Open (kokai) No. 9-271538 discloses, in the Examples and Comparative Examples, use of magnesium stearate as a dispersant which is added to a resin during coloring of the resin; and Japanese Patent Application Laid-Open (kokai) No. 11-178949 discloses, in the Examples and Comparative Examples, use of polyethylene wax as a dispersant which is added to a resin during coloring of the resin. Although a stearic acid metallic salt advantageously improves mold releasability, the metallic salt conceivably serves as a catalyst for decomposing a thermoplastic polyurethane material, and therefore, when the stearic acid metallic salt has been employed as a dispersant, considerable lowering of the heat resistance of the thermoplastic polyurethane material has been observed. Meanwhile, when polyethylene wax, exhibiting poor compatibility with a thermoplastic polyurethane material, has been employed as a dispersant, tendency of lowered dispersibility of pigments has been acknowledged.
In view of the foregoing, an object of the present invention is to provide a golf ball including a coverxe2x80x94having dimples thereon and being difficult to remove from a moldxe2x80x94which is formed from a thermoplastic polyurethane material exhibiting excellent mold releasability.
In order to achieve the above object, the present invention provides the following golf balls.
(1) A golf ball comprising a core and a cover therefor, wherein the cover is formed from a cover material containing a thermoplastic polyurethane material as a primary component and a fatty acid amide and/or a montan wax serving as dispersants.
(2) A golf ball according to (1), wherein the fatty acid amide is an alkylene bis(fatty acid amide).
(3) A golf ball according to (2), wherein the fatty acid amide is ethylene bis(stearamide).
(4) A golf ball according to any one of (1) through (3), wherein the montan wax is a montanic acid ester.
(5) A golf ball according to any one of (1) through (4), wherein the ratio by weight of the thermoplastic polyurethane material to the total amount of the fatty acid amide and the montan wax is 100:0.2 to 100:3.
(6) A golf ball according to any one of (1) through (5), wherein the cover material further contains, as a dispersant, a polyethylene wax having an average molecular weight of 1,000 to 10,000.
(7) A golf ball according to (6), wherein the ratio by weight of the thermoplastic polyurethane material to the total amount of the fatty acid amide, the montan wax, and the polyethylene wax is 100:0.2 to 100:3.0.